Fingerling release program, biomass trend and evolution of the condition factor of Caspian Kutum during the 1991-2011 period
This paper examines the changes in biomass and condition index in relation to stock enhancement of Kutum (Rutilus frisii kutum) in Iranian waters of the Caspian Sea during 1991-2011. Over this period, the condition factor of female and male Kutum showed decreasing trends. The average condition factor declined from 1.48 in 1991-1992 to 1.29 in 2010-2011 for females and from 1.40 in 1991-1992 to 1.20 in 2010-2011 for males. The instantaneous coefficient of natural mortality was estimated at 0.39/yr and the instantaneous coefficient of fishing mortality varied during the 20-years period between 0.20/yr to 1.04/yr. Biomass estimates of Kutum using biomass-based cohort analysis increased from about 34,700 mt in 1991-1992 to 40,900 mt in 1993-1994, declined to 28,800 in 2001-2002, increased to 56,900 in 2006-2007, and again declined to 42,600 in 2010-2011. The total number of released Kutum fingerlings increased from about 110 millions in 1991 to 400 millions pieces in 2010. Although there is a significantly positive correlation between fingerlings released and recruitments, it appears on the other hand that condition factor is adversely influenced by the number of fingerlings released. Moreover, over the recent period, biomass and catches do not seem to benefit so much from the increasing efforts in fingerlings release. We concluded that more research should be conducted to determine the desirable level of artificial propagation.